Batteries that are designed to withstand high temperatures, e.g., 270 degrees Fahrenheit (132 degrees Celsius), are typically vented to permit gas to escape and prevent excessive pressure in the battery. Battery vents, however, may permit electrolyte to escape when the battery is held with the vent below the level of the electrolyte, such as frequently occurs if the battery is attached to a hand-held powered device. Electrolyte may also escape during heat-sterilization if the battery is carelessly placed in an autoclave with the vent below the level of the electrolyte.
One approach for reducing the loss of electrolyte is to place a pressure-relief valve in the vent. Such a valve allows gas to be vented and reduces loss of electrolyte in normal operation, but does not prevent the discharge of electrolyte when the pressure in the battery is sufficient to open the relief valve. As a result of the gradual or sudden loss of electrolyte through a vent, the operation of vented batteries has been impaired before their components have worn out.
These problems with vented batteries have been particularly troublesome when the batteries are used to power orthopedic or other surgical instruments, which generally require high power and a high degree of reliability. These batteries are usually heat sterilized between each use, and have sufficient capacity to power the instrument during a potentially complicated and time-consuming surgical operation. It is especially important that sufficient electrolyte be retained to completely wet the electrodes throughout the life of the battery. Free or "excess" electrolyte is desirable in new batteries because electrolyte is typically lost as hydrogen and oxygen during charging of the battery. In addition, it is unacceptable for electrolyte to leak from the battery during a surgical operation.
Coassigned U.S. Pat. Nos. 4,728,876 and 4,892,794 describe orthopedic drive assemblies including a silver/zinc battery having potassium hydroxide electrolyte, and a vent assembly for venting gas from the battery cell. Those vent assemblies include a spring-biased ball valve in a vent passageway, and a curtain of hydrophobic polymeric material (e.g., polytetrafluoroethylene) across the passageway. U.S. Pat. Nos. 3,120,845; 3,173,417; 3,734,207; and 4,091,880 describe other battery-powered surgical instruments.
U.S. Pat. No. 3,909,302 discloses a vent cap for liquid acid batteries including a hydrophobic diaphragm (e.g., a microporous polytetrafluoroethylene sheet) across a vent passageway. U.S. Pat. Nos. 2,132,224; 3,497,104; 3,497,396; 4,584,248; and 4,636,446 disclose various battery vents or valves.